In the electronics industry, there is a need for regulating current. Many battery powered electronic devices include components for monitoring the current to and from a load, such as a battery pack. This is typically done by putting a sense resistor in the path between the battery pack and the load and also between the charging unit and the battery pack. A regulating circuit then measures the voltage across the sense resistors and limits the current to or from the battery pack accordingly. This limiting is typically performed by using a transistor in the current path from the battery pack to the load or charging unit and by causing the transistor's control terminal to be driven in response to the voltage across the sense resistor.
With battery and non-battery powered devices, often a main goal is to lose as little voltage as possible across the regulating circuitry when not limiting current. With typical current regulators, voltage is lost in two places: as current flows through the sense resistor and as current flows through the transistor. Consequently, when not limiting current in order to avoid wasting power or generating heat, it is desired that the resistance of the sense resistor and the resistance of the transistor be as low as possible for the area available for these components.
A transistor exhibits low resistance when its control terminal is held at a voltage appropriate to turn the transistor “full on.” In current regulating circuitry, this is often accomplished through the use of an amplifier coupled to the sense resistor with the amplifier output driving the control terminal of the transistor. With the appropriate choice of sense resistor and amplifier characteristics, the current regulating circuitry can be constructed such that for currents less than a regulation current limit, the amplifier drives the control terminal of the transistor to a full on state. As current through the sense resistor increases above the regulation current limit, the amplifier drives the control terminal of the transistor to reduce the current to the regulation current limit. This has the effect of providing little resistance when current passing through the regulating circuitry is less than the regulation current limit and provides the appropriate amount of resistance through the transistor to limit the current to the regulation current when a load tries to draw an excess amount of current.